I currently have iOS 5 installed, so is it better if I stay on it, as iOS 6 might slow it down further?

100% Stay with iOS5.

... I found that my iPhone 4 was a lot snappier with iOS 6 than with iOS 5. Just sayin. Sometimes poor performance is the result of added features, but sometimes there are bugs. iOS 5 ran like a dog on my iPhone 4 when compared to iOS 4 and iOS 6. Based on that, I'd say iOS 6 is better optimized than 5 was.

Just hold down the shift key/option key (mac) as you click restore (in itunes) and an explorer window will pop up where you can browse for the ipsw file.

The only IPSW file that will install is the one that Apple is actively signing for the device, which I believe is 6.1.2. Any previous version than that will require an SHSH stitch on the firmware file in order to have it install on your device w/o Apple's activation. The only way to get your SHSH blobs for your device is to either jailbreak it and use Cydia (which I don't think works anymore), or use TinyUmbrella. But, unless you have already collected these while Apple was doing the signing you will be SOL on ever getting them.

I still say just upgrade to the latest using iTunes update process and be done with it. I'm just letting you know in so-many-words that once you will most likely not have a way to get back to iOS 5. Don't let that detour you, however, as I still think iOS 6 is superior and worth it.

The only IPSW file that will install is the one that Apple is actively signing for the device, which I believe is 6.1.2. Any previous version than that will require an SHSH stitch on the firmware file in order to have it install on your device w/o Apple's activation. The only way to get your SHSH blobs for your device is to either jailbreak it and use Cydia (which I don't think works anymore), or use TinyUmbrella. But, unless you have already collected these while Apple was doing the signing you will be SOL on ever getting them.

I still say just upgrade to the latest using iTunes update process and be done with it. I'm just letting you know in so-many-words that once you will most likely not have a way to get back to iOS 5. Don't let that detour you, however, as I still think iOS 6 is superior and worth it.

Ah right, I've only every tried it with Beta and current OS...so that's why it'll let me go back...so yeh maybe don't try that then...but I'd still expect 6 to be as good if not better than 5.